In digital copiers and laser printers, in order to obtain high quality images more approaching conventional photography, high gloss images preferred are which are produced employing toner comprised of smaller diameter particles. In order to decrease the toner particle diameter, production of toner particles according to a polymerization method has received attention.
In copiers and the like, which utilize the electrophotographic process described above, it is required that a non-fixed toner image formed on a recording sheet is fixed to obtain a permanent image. Known as such fixing methods are a solvent fixing method, a pressure fixing method, and a thermal fixing method.
Employed as thermal fixing methods are a thermal contact type heating roller system which exhibits high heat efficiency as well as high safety and a film heating system which saves energy.
Thermal fixing apparatuses, which utilize the thermal contact fixing method, are comprised of a heating roller (being a fixing roller) and a pressure roller which is arranged to come into pressure contact with the fixing roller. The heating roller is comprised of a cored cylinder which comprises a heat lamp in its interior and a heat-resistant releasing layer is formed on its outer circumferential surface. The pressure roller is also compromised of a cored metal cylinder and a heat-resistant elastic material layer is formed on its outer circumferential surface. In conventional thermal fixing systems, supports such as plain paper sheets having a pre-fixed toner image are allowed to pass between both rollers to which a definite pressure is applied whereby fixing is carried out. A heating roller type fixing apparatus which utilizes the system exhibits higher thermal efficiency compared to those utilizing other fixing systems such as a hot air fixing system or an oven fixing system, whereby, at present, the fixing apparatus is most widely employed due to lower power consumption, higher production rate, and reduced fire hazard due to paper jamming.
However, in the contact heating roller system fixing apparatus, which employs a heating roller (being a rotating member for fixing), when transfer materials and toners are heated by a heating roller having a halogen heater in its interior, it is necessary to heat the fixing heating roller which has a high heat capacity. As a result, problems occur in which the life of the heating rubber roller is shortened due to the increase in temperature of the interior of the elastic layer, energy saving is adversely affected due to lower energy saving effects, and when printing is initiated, the time necessary for print initiation (being a warming-up period) increases due to an increase in time for warming up the fixing apparatus.
In recent years, in order to overcome the problems, the fixing unit described below has become commercially available. An on-demand type-fixing unit, comprised of a combination of a ceramic heater having a small heat capacity and film, has been realized. Namely, during stand-by, electricity is applied to the heater only for preliminary heating, and only when necessary, for example, as when paper sheets are supplied, electricity is supplied to the heater.
However, in the on-demand fixing system, employing a seamless film the problems described below have occurred. Electrostatic off-setting phenomena occur in which toner on a transfer material is electrostatically transferred onto the fixing roller or the fixing film. Further, the bleeding efficiency of releasing agents from toner decrease due to a lower fixing load, compared to the conventional fixing systems, whereby off-setting tends to occur. Particularly, fixing members are stained by slight off-setting, which is not detected on images, resulting in a decrease in life. Due to these drawbacks, fixing apparatuses employing the system have not been applied commonly to high speed printers or to high speed copiers.
Recently, in order to achieve energy saving systems, low temperature fixing techniques have been noted. However, when taking into account an increase in speed of printing process at low temperature, specifically off-setting properties are greatly affected depending on the characteristics of the employed toners. In the low temperature fixing system, problems occur in which when the toner comprised of conventional resins is employed, cold off-setting tends to occur due to excessively high elasticity. On the other hand, when as a characteristic of resins, elasticity is designed to be much lower than the necessary value, it becomes difficult to achieve compatibility between off-setting resistance and storage stability of toner due to coagulation and fusion of toner during storage.